The wording of this problem indicates that there was an illustration. Could you possibly share that illustration?
Working without an illustration:
If tan theta = 3, then tan theta = opp/adj = 3/1. This tells us that the opp side is 3 times as long as is the adj. side. Let x be the shorter side, i. e., let x represent the adjacent side; then y is the longer side and represents the opposite side.
Then y = 3x (the opp side is 3x the adj side in length).